An example of a known polyphenol glycoside is quercetin-3-glucoside (isoquercitrin), represented by the following formula (Non-Patent Document 1):
In such glycoside, a sugar is bound to the carbon at position 3 of quercetin via glycosidic linkage. However, glycoside in which a sugar is bound to the carbon at position 4 of quercetin via glycosidic linkage has not been known.
Known examples of a polyphenol compound analogous to quercetin include dehydroquercetin (taxifolin):
and leucocyanidin:
It has been known that, in plants, hydrogen binds to oxygen bound to the carbon at position 4 of dehydroquercetin, resulting in the generation of leucocyanidin and leading to the generation of cyanidin from leucocyanidin. That is, dehydroquercetin and leucocyanidin are intermediates used for cyanidin synthesis. Also, regarding dehydroquercetin and leucocyanidin, no glycoside in which a sugar is bound to the carbon at position 4 via a glycosidic linkage has been known.
Meanwhile, in accordance with changes in dietary habits and lifestyle of recent years, the number of diabetic patients is increasing. At present, the number of diabetic patients is as high as 7,000,000 in Japan, and such number could be as large as 15,000,000 when future diabetics are added. Diabetes is a metabolic disorder in which a prolonged hyperglycemic state caused by an insufficient level of insulin hormone activities is exhibited. A prolonged hyperglycemic state may result in development of various types of complications, such as nervous disorders, cataracts, renal disorders, retinopathy, arthrosclerosis, atherosclerosis, and diabetic gangrene.
Thus, inhibition of increases in blood sugar levels is thought to be involved in a method for treating or preventing diabetes. In this regard, many medical agents for treating and preventing diabetes and diabetic complications have so far been developed.
Examples of such medical agents include an α-glucosidase inhibitor that inhibits digestion and absorption of carbohydrates to prevent the blood glucose level from becoming elevated. Voglibose and acarbose are known as representative α-glucosidase inhibitors.
While these agents have remarkable effects, they impose various side effects on patients, such as a feeling of fullness upon ingestion, induction of a hypoglycemic state due to the combined use thereof with other hypoglycemic agents, and nausea or headache. In order to overcome such drawbacks, agents made from natural ingredients, which have mild effects and are free from problems related to side effects, have been developed. For example, an extract of Japanese basil (Patent Document 1), an extract of yerba mate leaves (Patent Document 2), an extract of Apocynum venetum leaves (Patent Document 3), and an extract of eriobotryae folium (Patent Document 4), are known as α-glucosidase inhibitors made from natural ingredients, although the number of such agents that have been provided is not sufficiently large.
Meanwhile, active oxygen has been known to have adverse influences upon living bodies. Examples of such adverse influences upon living bodies include aging, carcinogenesis, and development of blemishes or freckles. In addition, active oxygen has been known to cause deterioration of cosmetics, beverages, and foods. As an antioxidant that removes active oxygen, ascorbic acid (vitamin C) or the like is used in cosmetics, beverages, and foods. In recent years, an antioxidant with improved safety that is derived from a natural product has been awaited.    Patent Document 1: JP Patent Publication (Kokai) No. 2000-102383 A    Patent Document 2: JP Patent Publication (Kokai) No. 2003-146900 A    Patent Document 3: JP Patent Publication (Kokai) No. 2002-053486 A    Patent Document 4: JP Patent Publication (Kokai) No. 2003-128571 A    Non-Patent Document 1: Chem. Pharm. Bull. 49 (2)151-153 (2001)